Electrophotography is used exceedingly widely in recent years in the fields of copiers, laser beam printers, facsimile telegraphs, etc., because this technology has an advantage that both high-speed printing and high print quality can be obtained. Widely known as electrophotographic photoreceptors for use in these electrophotographic apparatus are those employing an inorganic photoconductive material such as, e.g., selenium, a selenium-tellurium alloy, a selenium-arsenic alloy, or cadmium sulfide. On the other hand, investigations on electrophotographic photoreceptors employing an organic photoconductive material which have advantages of low cost and superiority in producibility and disposability over the electrophotographic photoreceptors employing such inorganic photoconductive materials are becoming intensive. Of these organic photoreceptors, the organic multilayer photoreceptors of the function separation type comprising a charge generating layer which generates charges upon exposure to light and a charge transport layer which transports the charges are superior in electrophotographic characteristics including sensitivity, electrification characteristics, and stability thereof in repeated use. Various organic photoreceptors of such multilayer type have been proposed and put to practical use.
Organic photoreceptors of the single-layer type not only are advantageous in producibility and production cost but have an advantage that they enable positively charging systems (ozone generation reduction, evenness of electrification). However, since the single-layer organic photoreceptors presently have a problem that they are inferior in electrical performances to multilayer photoreceptors, there is yet much room for investigations and developments.
Although some of the organic photoreceptors developed so far have sufficient performances concerning such electrophotographic characteristics, they have poor resistance to mechanical external force because they comprise organic photoconductive materials. Specifically, the photoreceptor surface undergoes wearing, marring, etc. due to direct loading by toner, developing agent, paper, and a cleaning member, and recently by a roll or another means which comes into direct contact with the photoreceptor to electrify it. In addition, undesirable deposition such as toner filming occurs on the photoreceptor surface. Such troubles on the photoreceptor surface cause the problem of image defects. Other problems are that the ozone, nitrogen oxide, etc. generated by corona discharge cause a deterioration of the surface layer of the photoreceptor, and that a paper dust released by receiving paper and other dust particles adhere to and accumulate on the photoreceptor surface to cause image diffusion in a high-humidity atmosphere. The service life of the photoreceptor is limited by these problems.
With the spread of digital copiers or printers, in which exposure is conducted with a semiconductor laser, photoreceptors have come to be required to have the maximum sensitivity within the near infrared wavelength region for the semiconductor laser (780-830 nm). Moreover, since the time necessary for one process consisting of charging, exposure, development, transfer, cleaning, and erase tends to become shorter with the trend toward color electrophotography, higher-speed processing, and size reduction, photoreceptors are also required to have both quicker response to light and long-term electrical stability. Thus, the photoreceptors have come to be used in a more complicated process under higher stresses, and are hence required to have even higher durability.
Various kinds of materials for use as binder resins for the photosensitive layers of these electrophotographic photoreceptors have conventionally been investigated. For example, use of various modified polycarbonate resins as a binder resin for a photoreceptor surface layer has been proposed (JP-B-2-57300, JP-A-62-247374, JP-A-63-148263, JP-A-1-177551, JP-A-2-254458, JP-A-2-254459, JP-A-3-63651, JP-A-3-150571, JP-A-4-179961, JP-B-5-3584, JP-A-5-80548, and JP-A-5-142800). (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication," respectively.)
Use of those proposed resins as a binder resin for a photosensitive layer yields electrophotographic photoreceptors having relatively satisfactory durability. However, this durability is still insufficient. Namely, the mechanical strength of coating films formed using those resins is not always sufficient, so that the surface of the photosensitive layer wears during long-term repeated use in a copier. As a result, the photoreceptor suffers a change in its film thickness, a decrease in electrification potential, and a change in sensitivity, resulting in copies having fogging or a reduced density. Furthermore, the surface marring of the photoreceptor causes image defects. On the other hand, even when a resin having better wearing resistance is used, its electrical characteristics are insufficient and use of such a resin having better abrasion resistance has a problem that undesirable deposition such as toner filming occurs to cause image defects. In addition, if a binder resin which is soluble only in low-boiling organic solvents such as methylene chloride and tetrahydrofuran is used for forming a photosensitive layer, application of the coating fluid containing this resin has problems that the coating film surface adsorbs moisture in the surrounding air to cause the phenomenon called blushing or to result in an orange-peel surface. Although a high-boiling organic solvent may be mixed in order to eliminate these problems, this causes the coating fluid to gel in an early stage, resulting in impaired production stability.
The compatibility of charge transport materials with binder resins is also important. It has been made clear that a charge transport material having insufficient compatibility partly undergoes crystallization, precipitation, etc. to produce a considerable influence on electrical characteristics and image quality. Even in the case of a combination having good compatibility as in a charge transport layer comprising a combination of the copolycarbonate disclosed in JP-B-5-3584 with a benzidine compound as a charge transport material, there is a drawback that image quality is impaired due to substances generated by electrical discharge for electrification. In addition, this charge transport layer, when used in electrophotographic apparatus employing a thinner drum and having a higher processing speed, is required to be further improved in electrical characteristics.